To examine the value of a variable, use C-h v
describe-variable). This reads a variable name using the
minibuffer, with completion, and displays both the value and the
documentation of the variable. For example,
C-h v fill-column <RET>
displays something like this:
fill-column is a variable defined in `C source code'. fill-column's value is 70 Automatically becomes buffer-local when set. This variable is safe as a file local variable if its value satisfies the predicate `integerp'. Documentation: Column beyond which automatic line-wrapping should happen. Interactively, you can set the local value with C-x f. You can customize this variable.
The line that says “You can customize the variable” indicates that this variable is a user option. C-h v is not restricted to user options; it allows non-customizable variables too.
The most convenient way to set a specific customizable variable is with M-x set-variable. This reads the variable name with the minibuffer (with completion), and then reads a Lisp expression for the new value using the minibuffer a second time (you can insert the old value into the minibuffer for editing via M-n). For example,
M-x set-variable <RET> fill-column <RET> 75 <RET>
fill-column to 75.
M-x set-variable is limited to customizable variables, but you can set any variable with a Lisp expression like this:
(setq fill-column 75)
To execute such an expression, type M-: (
and enter the expression in the minibuffer (see Lisp Eval).
Alternatively, go to the *scratch* buffer, type in the
expression, and then type C-j (see Lisp Interaction).
Setting variables, like all means of customizing Emacs except where otherwise stated, affects only the current Emacs session. The only way to alter the variable in future sessions is to put something in your initialization file (see Init File).